An Introduction to Environmental Chemistry

122 Chapter Four

Box 4.14 Physical and chemical properties that dictate the fate of

organic contaminants

Polarity

Polarity is a measure of how skewed the

electron distributions are within a molecule.

We have seen previously that water (H 2 O) has

an oxygen atom with a negative dipole

(imbalance in charge) and two hydrogen

atoms with positive dipoles (Box 4.1). These

dipoles arise because oxygen is more

electronegative (Box 4.2) and therefore draws

more of the electron density in the

oxygen–hydrogen bonds closer to itself.

Conversely, hydrogen being more

electropositive assumes a positive dipole in

the molecule. Other molecules display

polarity as shown below (Fig. 1).

Solubility

The solubility of a compound depends on its

polarity. If water is the solvent, the

compound will need to be of similar polarity

to be dissolved in it, for example ethanol

(CH 3 CH 2 OH). In ethanol the OH-group is polar

and therefore the molecule has polar

character (Fig. 1b). If the compound is non-

polar, for example ethane, it will not dissolve

in polar water (Fig. 1c) because opposites do

not mix. Ethanol will, however, dissolve in a

non-polar solvent, for example hexane. Thus,

the term ‘solubility’ should always be used

with clarification of the solvent, for example

‘aqueous solubility’. Solubility is expressed as

the mass of a substance that will dissolve in a

given volume of solvent, for example mg l-^1.

Hydrophobicity

The hydrophobicity of a compound is a

measure of its affinity for water. If a

compound will not readily partition into the

aqueous phase it is known as hydrophobic

(fearing water). A converse term, often used

when talking about organic compounds, is

lipophilicity. If a compound is lipophilic it

loves lipid (fat). These terms are used

interchangeably when the environmental

fate of organic contaminants is discussed.

Hydrophobicity is measured as a partition

coefficient between octanol/water (KOW).

Octanol is chosen to represent lipid (fat)

because it is an experimentally reproducible

compound. KOWvalues are determined by

allowing the compound of interest to

equilibrate between the two phases: water

and octanol. After equilibration the

concentrations in each phase are determined

and their ratio calculated (Fig. 2). Most

organic compounds are very hydrophobic

such that KOWvalues are often in the range

10 000 to 1000 000. In order to work with

smaller numbers the log of the KOWvalue

is usually used, such that the values are

typically between 4 and 6.

Vapour pressure and volatility

Water vapour condenses to liquid water

when cooled. At a constant pressure liquid

water appears abruptly at a specific

temperature and the pressure is known as

the vapour pressure for this temperature. The

vapour pressure of a compound is a measure

of its volatility, i.e. its tendency to evaporate

into the gas phase. A compound with high

vapour pressure is described as volatile, while

a compound with low vapour pressure is

described as non-volatile. The vapour

(c)

H 3 C – CH 3

Ethane

(b)

H 3 C – C – O – H

H

H

Ethanol

(a)

O

H H

d–

d+ d+

Water

d– d+

Fig. 1Dipole distribution in (a) water (polar), (b) ethanol (polar) and (c) ethane (non-polar).

(continued)